We continue to study the distribution of brain neurotransmitter or drug receptors by our newly developed autoradiographic method. Quantitation of receptor distribution with tritium-sensitive film is now also possible. Specifically, chemically coded neuronal pathways can be surmised from the concordance of receptor distributions and autoradiographic track tracing techniques. Furthermore, adjacent sections can be incubated under different conditions with different ligands to reveal information about receptor subtypes.